Mill Creek (aka Big Mill Creek) IA 01-TRK-115

Alternate name(s) for this segment: Big Mill Creek

confluence with Little Mill Cr. to confluence with unnamed tributary in S1 T86N R3E Jackson Co.

Open Data Collection Period Modal

For purposes of developing stream/river water quality assessments for integrated reporting, three years of water quality data are typically used for both conventional pollutant parameters (e.g., indicator bacteria) and the less frequently monitored toxic parameters (e.g., toxic metals). Since the 2004 IR cycle, DNR has used a three-year data gathering period for streams, rivers, and beaches. For most assessments, the use of three years of data increases the number of samples upon which the decision on use support is based and helps address the problem of weather-related year-to-year fluctuations in water quality.

Due to the lower sampling frequency in Iowa’s ambient lake, biological, fish kill, and fish tissue monitoring programs, five years of data are used for developing IR assessments and for identifying Section 303(d) listings.

Data Type	Data Timeframe
Beach Sampling Data	CY 2020 - 2022
Stream/River Water Quality Data	CY 2020 - 2022
Lake Water Quality Data	CY 2018 - 2022
Biological Sampling Data	CY 2018 - 2022*
Fish Kill Data	CY 2018 - 2022
Fish Tissue Sampling Data	CY 2018 - 2022
Continuous Dissolved Oxygen Sampling Data	CY 2018 - 2022
*Biological sites sampled in both 2017 and 2021 were also assessed.

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Cycle: 2024
Release Status: Final
Overall IR: 2 - Some of the designated uses are met but there is insufficient data to determine if remaining designated uses are met.
Trend: Unknown
Created: 11/1/2023 8:12:29 AM
Updated: 1/9/2024 11:39:10 AM

Use Support

Class A1

Presumptive Use

Recreation - Primary contact

Not Assessed

Class A2

Recreation - Secondary contact

Not Assessed

Class BCW1

Aquatic Life - Cold Water Type 1

WINOFI

Biological: low aquatic macroinvertebrate IBI

Support Level

Further Investigation Needed

Impairment Code

3b-c - Use potentially biologically impaired based on calibrated IBI metrics.

Cause Magnitude

Moderate

Status

Continuing

Source

Unknown: Source Unknown

Source Confidence

Low

Cycle Added

2012

Impairment Rationale

Low Biotic Index

Data Source

Biological monitoring: Iowa DNR WQMA

Class HH

Human Health -

Fully Supported

Impairment Delistings

No delistings for this assessment cycle.

Documentation

Assessment Summary

The Class BCW1 use was assessed as "WINOFI" based on biological sampling data. The Class HH use (fish consumption) was assessed as "fully supported" based on fish tissue monitoring data. All other designated uses were "not assessed."

Assessment Explanation

Data Sources:

Data Source	Data Source ID	Data Type	Data Age	Site ID	Site Name	Site Description
Iowa DNR	6	BIO	See Below	663	Big Mill Creek	Bellevue – BigMil1
Iowa DNR	6	FT	See Below	1361	Big Mill Creek	Big Mill Creek WMA

Presumptive Class A1 - No Data

Class A2 - No Data

Class BCW1 - Biological Sampling:

Benthic Macroinvertebrates:

BioNet Site ID	Date	Drainage Area (mi2)	Ecoregion	Major Drainage/Thermal	BM Sample Gear	CBI score	CBI Class	CBI BIT	CBI Pass/Fail
663	8/22/22	13	52b	MS/CW	Hess	36	NA	60	Fail

Overall Assessment:

Community	Assessment Type	Assessment Result	Support Level	IR Category	Reason
Benthic Macroinvertebrates	Evaluated	Fail (0/1)	WINOFI	3bc	Assessment was evaluated because it was based on a single sample.

Class BCW1 – Trout Natural Reproduction:

In contrast to the benthic macroinvertebrate data, based on the January 2024 summary of trout reproduction in Iowa streams prepared by the DNR Fisheries Bureau, the Class BCW1 use of (Big) Mill Creek was assessed as "fully supported.” According to the updated summary, this stream was placed in Category I: streams that exhibit consistent natural reproduction and maintain a wild population of one or more trout species without any stocking.

Class HH - Fish Tissue Sampling:

Site ID	Biopart	Year	Species	# in sample / # of samples	Avg Length (cm)	(Avg) Hg (ppm)
1361	tissue plug	2019	brown trout	10	32.6	0.033
Support Level	Full

Monitoring and Methods

Assessment Key Dates

8/22/2022

Biological Monitoring

1/1/2024

Biological Monitoring

7/1/2019

Fish Tissue Monitoring

Methods

260

Fish tissue analysis

315

Regional reference site approach

320

Benthic macroinvertebrate surveys

330

Fish surveys